Dielectrically supported helix derived slow wave circuit

ABSTRACT

A dielectrically supported helix derived slow wave circuit and microwave tube using same is disclosed. A topologically equivalent ring and bar slow wave circuit is supported from mutually opposed concave trough-shaped surfaces of a serpentineshaped dielectric support structure, as of ceramic. The outer edges of the serpentine support structure are bonded to the inner surface of a metallic barrel structure surrounding the slow wave circuit. An array of metallic fins extend from the barrel into a region adjacent the slow wave circuit and in between adjacent ring portions of the ring and bar circuit for decreasing the dispersiveness of the ring and bar circuit at the low frequency end of the operating range whereby the useful bandwidth of the tube is greatly increased.

United States Patent Scott et al.

[54] DIELECTRICALLY SUPPORTED HELIX DERIVED SLOW WAVE CIRCUIT [72] Inventors: Allan W. Scott; Yukio Hlramatsu, both of Los Altos, Calif.

[73] Assignee: Varlan Associates, Palo Alto, Calif.

[22] Filed: Dec. 14, 1970 [2]] Appl. No.: 97,687

[451 Apr. 4, 1972 2,925,515 2/l960 Peter ..3l5/3.5 3,505,730 4/1970 Nelson ..29/600 Primary Examiner-Herman Karl Saalbach Assistant Examiner-Saxfield Chatmon, Jr. Attorney-Stanley Z. Cole [5 7] ABSTRACT A dielectrically supported helix derived slow wave circuit and microwave tube using same is disclosed. A topologically equivalent ring and bar slow wave circuit is supported from mutually opposed concave trough-shaped surfaces of a serpentine-shaped dielectric support structure, as of ceramic. The outer edges of the serpentine support structure are bonded to the inner surface of a metallic barrel structure surrounding the slow wave circuit. An array of metallic fins extend from the barrel into a region adjacent the slow wave cir cuit and in between adjacent ring portions of the ring and bar circuit for decreasing the dispersiveness of the ring and bar circuit at the low frequency end of the operating range whereby the useful bandwidth of the tube is greatly increased.

9 Claims, 6 Drawing Figures Patented April 4, 1972 WITHOUT VANES WITH VANES FIG.3

U S SU R A m M V A E N R H N T K L A A I Y B GOVERNMENT CONTRACT The invention herein described was made in the course of 5 work under a contract with the Department of the United States Air Force.

DESCRIPTION OF THE PRIOR ART Heretofore, it has been proposed to support a ring and bar structure from a surrounding metallic barrel via the intermediary of an array of metallic posts interconnecting the barrel with the bar portions of the ring and bar circuit. In such a tube, the support posts had a length on the order of a quarter wave length to provide high impedance to the microwave circuit. In addition, electrically conductive loading combs, consisting of arrays of metallic vanes were connected to the barrel and projected into the region between adjacent ring portions of the ring and bar slow wave circuit for decreasing the dispersiveness of the circuit at the low frequency end of the operating range. Such a slow wave circuit is disclosed in an article titled High Power Applications of the Connected Ring Structures in Traveling Wave Tubes," by Walter Revis Ayers appearing in ASTIA AD Report 207168, ML Report No. 554 of 1958, page 50.

While the loading combs of the cited prior art served to decrease the dispersiveness of the circuit, the stub supported circuit was still a relatively dispersive circuit having a bandwidth of substantially less than a half an octave.

It is also known, from the prior art, to support ahelix derived circuit, such as a ring and bar circuit, from a ceramic comb or serpentine-shaped ceramic structure which is bonded to the slow wave circuit and to the barrel of the slow wave tube for increasing the thermal capacity of the circuit. Such slow wave circuits and slow wave tubes are disclosed and claimed in: U.S. Pats. Nos. 3,505,730 issued Apr. 14, 1970, US. Pat. No. 3,508,108 issued Apr. 21, 1970 and in US. Pat. application Ser. No. 8,793, filed Feb. 5, 1970, all of which are assigned to the same assignee as the present invention. While such dielectrically supported helix derived slow wave circuits have increased thermal capacity and provide relatively wide band operation, as of 20 percent, it is desired to increase the bandwidth of such dielectrically supported helix derived slow wave circuits to more than 40 percent and preferably to a full octave of bandwidth.

SUMMARY OF THE PRESENT INVENTION The principal object of the present invention is the provision of an improved dielectrically supported helix derived slow wave circuit and tubes using same.

One feature of the present invention is the provision, in a dielectrically supported helix derived slow wave circuit, of an array of electrically conductive vanes interposed between adjacent dielectric support vanes of the dielectric support, such electrically conductive vanes being electrically coupled to the metallic barrel of the tube to provide a microwave ground closely spaced between periodic elements of the slow wave circuit for decreasing the dispersiveness of the slow wave circuit at the low frequency and of its operating range.

In another feature of the present invention, the root portions of the conductive vanes are joined to barrel structure.

In another feature of the present invention, the dielectric slow wave circuit support is serpentine-shaped having an array of vane shaped dielectric members interconnected to form a meandering serpentine-shaped structure, the electrically conductive vane members interdigitated with the dielectric support vanes and the serpentine shaped dielectric support being bonded to a correspondingly serpentine-shaped slow wave circuit portion of a ring and bar circuit.

In another feature of the present invention, the electrically conductive vane members are spaced apart from the adjacent dielectric vane members.

In another feature of the present invention, the thickness of each electrically conductive vane member is approximately equal to one-half the spacing between adjacent dielectric support vanes.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic perspective for shortened view of a prior art microwave tube,

FIG. 2 is a plot of phase velocity versus frequency depicting the dispersive characteristic of the prior art tube and of the tube employing features of the present invention,

FIG. 3 is a sectional view of the prior art structure of FIG. 1 taken along line 3-3 in the direction of the arrows as modified to incorporate features of the present invention,

FIG. 4 is a view of the structure of FIG. 3 taken along line 4-4 in the direction of the arrows,

FIG. 5 is a sectional view of the structure of FIG. 4 taken along line 5-5 in the direction of the arrows, and

FIG. 6 is an enlarged detailed view of the portion of the structure of FIG. 5 delineated by line 6-6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1, there is shown a prior art microwave tube 1. The tube 1 includes an electron gun assembly 2 for forming and projecting a beam of electrons 3 over an elongated beam path to a beam collector structure 4 disposed at the terminal end of the beam for collecting and dissipating the energy of the beam. A helix derived slow wave circuit 5 is disposed intermediate the gun 2 and the collector 4 along the beam path for cumulative electromagnetic interaction with the beam to produce output microwave energy which is extracted from the circuit 5 via a conventional output coupler, not shown, in the manner well known in the art. Such a slow wave tube is disclosed in the aforecited U.S. Pats. Nos. 3,508,108 and 3,505,730, and in the aforecited US. Pat. application No. 8,793.

The 0 slow wave circuit 5 comprises a topologically equivalent ring and bar helix derived microwave circuit including a pair of opposed meander line portions 6 disposed in transverse registration upon the concave trough surfaces of a pair of serpentine-shaped ceramic insulator structures 7, as of alumina or beryllia ceramic. In a preferred embodiment, the ring and bar slow wave circuit portions 6 are conveniently bonded to the dielectric support 7 by sputter depositing a relatively thin layer of molybdenum or tungsten to a thickness of 400 to 10,000 angstroms. The sputter deposited metal forms a tightly adhering layer. Rhodium is then plated over the base layer to a thickness, as of 0.0002-0.0003 inch. The serpentine-shaped dielectric supports 7 are each brazed to a plate 8 having a coefficient of thermal expansion matching that of the ceramic. Such a suitable material includes Elkonite (copper impregnated tungsten matrix). The plates 8 are in-turn brazed into a copper barrel 9 of rectangular cross-section forming the vacuum envelope of the tube.

The microwave circuit 5 of the prior art, as shown in FIG. I, has a dispersive characteristic as indicated by curve 11 of FIG. 2, which is characterized by decreasing phase velocity with increasing frequency. It is desired to decrease the dispersiveness of the circuit in order to improve the operating bandwidth of the tube. The prior art tube, at X-band, provided approximately 20 percent bandwidth and it is desired to increase this bandwidth to 40 percent or more.

Referring now to FIGS. 3-6, there is shown the helix derived microwave circuit 12 of the present invention. The circuit 12 is substantially identical to that previously described with regard to FIG. 1 with the exception that an array of electrically conductive vanes 13, as of molybdenum, are interposed in the space between adjacent vane portions 14 of the dielectric circuit support structure 7. The copper vanes 13 are bonded, as by brazing to the plate portion 8 of the barrel structure 9. The inner tips of the conductive vanes 13 have a concave curve conforming to the outer curvature of the microwave circuit 12.

The conductive vanes 13 are axially spaced in the direction of the beam, from the adjacent dielectric vanes 14 by a small gap :1 and preferably extend as close as possible to the outer surface of the circuit 12. The spacing from the inner tips of the vanes 13 to the adjacent slow wave circuit portion 6 should be less than the spacing 8 between adjacent periodic elements of the circuit 6. The axial gap d allows the interaction impedance of the slow wave circuit to be maintained at a relatively high value. If the electrically conductive vane 13 occupies too much of the gap g between adjacent dielectric vanes 14, the conductive vane 13 unduly increases the stored energy around the outside of the circuit, thereby reducing the interaction impedance. in a preferred embodiment, the thickness w of the metallic vane 13 is approximately equal to one-half of the gap g between the adjacent dielectric vane members 14.

The electrically conductive vane 13 serves to prevent the wave energy traveling along the meandering slow wave circuit from fringing across the gap between adjacent periodic elements of the slow wave circuit, i.e., across the gap g, especially at low frequency end of the operating bandwidth where the wavelength is relatively long compared to the spacing g. The improved dispersive characteristic for the vane loaded circuit 12 is as shown by curve 16 of FIG. 2. This improved dispersive characteristic greatly increases the operating bandwidth of the tube and operating bandwidths in excess of 40 percent have been achieved while maintaining the relatively high power handling capability of the ceramic supported helix derived slow wave circuit.

In a typical X-band example, the diameter of the beam hole through the circuit 12 is approximately 0.078 inch, the period of the microwave circuit from the center of one ring to the center of the adjacent ring is 0.06 inch, the width of the ribbon conductor 6 is 0.015 inch and the gap g between adjacent ring and dielectric vane portions is 0.0 l inch. The thickness of the conductive vanes 13 is 0.005 inch and the vanes 13 have a height and width of 0.120 inch and 0.120 inch, respectively. The ribbon-shaped conductor 6 of the circuit 12 has a thickness of 0.003 inch.

Since many changes could be made in the above construction and many apparently widely different embodiments of this invention could be made without departing from the scope thereof, it is intended that all matter contained in the above description or shown in the accompanying drawings shall be interpreted as illustrative and not in a limited sense.

What is claimed is:

1. In a microwave tube, electron gun means for projecting a beam of electrons over an elongated beam path, metallic periodic slow wave circuit means disposed along the beam path for cumulative electromagnetic interaction with the beam to produce microwave output energy, electrically conductive barrel means surrounding said slow wave circuit,

' dielectric support means interposed between said barrel means and said slow wave circuit for supporting said slow wave circuit relative to said barrel means, said dielectric support means including a plurality of dielectric vane portions spaced apart along the direction of the beam with the plane of each vane being generally transversely disposed of the beam path, each of said dielectric vane portions having a concave inner tip portion abutting said slow wave circuit and an outer root portion, and an array of electrically conductive vane members interposed between adjacent dielectric vane portions and electrically coupled to said conductive barrel means to provide a microwave ground disposed between periodic elements of said slow wave circuit means for decreasing the dispersiveness of said slow wave circuit means.

2. The apparatus of claim 1 including, means for bonding said dielectric support means to said slow wave circuit and to said conductive barrel, respectively, for providing thermally conductive joints between said dielectric support means and said slow wave circuit andbarrel, respectively.

3. The apparatus of claim 1 wherein said conductive vanes are joined along their side edges to said barrel.

4. The apparatus of claim 1 wherein said slow wave circuit and said dielectric support means each include a serpentineshaped member, said serpentine-shaped slow wave circuit member having a serpentine-shaped outer convex surface, said dielectric support member having a serpentine-shaped concave face joined to said outer surface of said slow wave circuit in transverse registration therewith.

5. The apparatus of claim 4 including, means for bonding an outer serpentine-shaped face of said serpentine-shaped dielectric support member to said surrounding barrel structure.

6. The apparatus of claim 4 wherein said electrically conductive vanes each include an outer root portion joined to said barrel and an inner concave tip portion terminating outside the outer circumference of said periodic slow wave circuit, the spacing from said tip portion of the said electrically conductive vane to the adjacent periodic element of said slow wave circuit being less than the space g between adjacent periodic elements of said slow wave circuit.

7. The apparatus of claim 6 wherein said periodic slow wave circuit comprises a topological equivalent of a ring and bar slow wave circuit.

8. The apparatus of claim 6 wherein each of said electrically conductive vane members is spaced apart from the adjacent dielectric vane portion in the direction of the beam.

9. The apparatus of claim 8 wherein the thickness of each of said conductive means is approximately equal to one-half the spacing between adjacent dielectric vane portions. 

1. In a microwave tube, electron gun means for projecting a beam of electrons over an elongated beam path, metallic periodic slow wave circuit means disposed along the beam path for cumulative electromagnetic interaction with the beam to produce microwave output energy, electrically conductive barrel means surrounding said slow wave circuit, dielectric support means interposed between said barrel means and said slow wave circuit for supporting said slow wave circuit relative to said barrel means, said dielectric support means including a plurality of dielectric vane portions spaced apart along the direction of the beam with the plane of each vane being generally transversely disposed of the beam path, each of said dielectric vane portions having a concave inner tip portion abutting said slow wave circuit and an outer root portion, and an array of electrically conductive vane members interposed between adjacent dielectric vane portions and electrically coupled to said conductive barrel means to provide a microwave ground disposed between periodic elements of said slow wave circuit means for decreasing the dispersiveness of said slow wave circuit means.
 2. The apparatus of claim 1 including, means for bonding said dielectric support means to said slow wave circuit and to said conductive barrel, respectively, for providing thermally conductive joints between said dielectric support means and said slow wave circuit and barrel, respectively.
 3. The apparatus of claim 1 wherein said conductive vanes are joined along their side edges to said barrel.
 4. The apparatus of claim 1 wherein said slow wave circuit and said dielectric support means each include a serpentine-shaped member, said serpentine-shaped slow wave circuit member having a serpentine-shaped outer convex surface, said dielectric support member having a serpentine-shaped concave face joined to said outer surface of said slow wave circuit in transverse registration therewith.
 5. The apparatus of claim 4 including, means for bonding an outer serpentine-shaped face of said serpentine-shaped dielectric support member to said surrounding barrel structure.
 6. The apparatus of claim 4 wherein said electrically conductive vanes each include an outer root portion joined to said barrel and an inner concave tip portion terminating outside the outer circumference of said periodic slow wave circuit, the spacing from said tip portion of the said electrically conductive vane to the adjacent periodic element of said slow wave circuit being less than the space g between adjacent periodic elements of said slow wave circuit.
 7. The apparatus of claim 6 wherein said periodic slow wave circuit comprises a topological equivalent of a ring and bar slow wave circuit.
 8. The apparatus of claim 6 wherein each of said electrically conductive vane members is spaced apart from the adjacent dielectric vane portion in the direction of the beam.
 9. The apparatus of claim 8 wherein the thickness of each of said conductive means is approximately equal to one-half the spacing between adjacent dielectric vane portions. 